Trace Element Organic Speciation along the US GEOTRACES Pacific Meridional Transect

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In many areas of the ocean microbes live in an environment that has very little of the nutrients they need to grow and thrive. In particular, nutrients with nitrogen (N), phosphorus (P), and iron (Fe), needed for the synthesis of proteins and nucleic acids, are in short supply. Iron is supplied to the ocean by dust blowing off the continents, and in areas remote from land, microbial life is limited by the very low concentrations of iron dissolved in seawater. To extract iron from their surroundings, some microbes synthesize and release organic compounds called siderophores into their environment. Siderophores are specifically designed to bind iron and transport it back into the cell. But only recently have we had the technology to measure siderophores in seawater, and we do not know where or when they are used, or which microbes are making and using them. The study proposed here is designed to address all of these questions. We will measure siderophores in the Pacific Ocean along a track from Alaska to Tahiti. The distribution of siderophores will be compared with data from other measurements (nutrients, cell numbers, genomics) to understand how microbes are able to live in very low iron environments, and how they can use organic compounds to extract metals from seawater. The study will also allow us test and improve the technology of measuring iron and other metals (mercury, copper, and cadmium, for example) bonded to organic compounds in other environmental samples, such as ground-waters, lakes and rivers, which is important for monitoring the toxicity of metal contaminants. Nearly all iron dissolved in the ocean is complexed by strong organic ligands of unknown composition. The effect of ligand composition on microbial iron acquisition is poorly understood, but amendment experiments using model ligands show they can facilitate or impede iron uptake depending on their identity. Here we propose to measure the molecular speciation of a suite of bioactive trace element (iron, copper, cobalt, nickel, and zinc) ligands (TE-Ls) in particulate and dissolved organic matter across the US GEOTRACES Pacific Meridional Transect (PMT). We will use high pressure liquid chromatography coupled to inductively coupled plasma mass spectrometry to detect and quantify TE-Ls, and companion electrospray ionization mass spectra to identify and characterize organic ligands. The PMT will cross five different biogeochemical provinces: shelf/slope, subarctic high nutrient/low chlorophyll (HNLC), North Pacific oligotrophic gyre, equatorial HNLC, and South Pacific oligotrophic gyre. The cruise track further intersects at least three different subsurface features, the subarctic and equatorial particle veils, oxygen deficient waters, and mid depth hydrothermal plumes. We expect the unique physical, chemical, and biological properties that characterize these regimes and features will lead to very different TE-L distributions across and down the water column. TE-L molecular speciation measurements will enable us to better integrate datasets of trace element distribution with metagenomic datasets of nutrient-driven changes in microbial metabolism across some of the Earth?s major biomes.